1. Field of the Invention
The present invention relates to a spectrometry device, such as an ultraviolet-visible spectrophotometer (UV), that is used as an independent measurement device or as a detector of a measurement device such as a liquid chromatograph, a liquid chromatograph including the spectrometry device as a detector, and a wavelength calibration method of the spectrometer. The liquid chromatograph may be a high-performance liquid chromatograph.
2. Description of the Related Art
Some spectrometry devices include a diffraction grating as a wavelength dispersion element, and by rotating the diffraction grating, select the wavelength of diffracted light that is to emerge from an exit slit. A pulse motor for rotating the diffraction grating is provided, and a predetermined number of control pulses are sent from a control unit to the pulse motor to thereby rotate the diffraction grating by a predetermined angle and to select the wavelength of the diffracted light.
The wavelength of diffracted light to emerge from the exit slit and the number of control pulses for rotating the diffraction grating are theoretically determined in advance, and the control unit sends the number of control pulses corresponding to the target wavelength to the pulse motor.
Depending on the accuracy of attachment of the diffraction grating or the accuracy of processing of parts, a theoretically determined wavelength may not be obtained even if a predetermined number of control pulses are sent to the pulse motor. Thus, calibration is performed by using an emission line spectrum whose wavelength is known.
Calibration of the rotation position of the pulse motor for rotating the diffraction grating is performed, for example, by searching for an emission line in the spectrum of a deuterium (D2) lamp and defining its position as a reference position 656.1 nm on the long-wavelength side, and searching for an emission line in the spectrum of a mercury lamp and defining its position as a reference position 253.7 nm on the short-wavelength side. The position of an emission line is already determined based on the physical quantity, and thus, there is no individual variability, and the reference positions for calibration do not change.
Use of the mercury lamp that is used for determining the reference position on the short-wavelength side will possibly be prohibited in the future due to restrictions by “Restriction of Hazardous Substances (RoHS)”. Accordingly, as an alternative to the emission line wavelength of the mercury lamp, use of the peak wavelength of Ho absorption spectrum of a holmium (Ho) glass filter has been considered. The absorption peak of Ho is at 241.7 nm (NIST (National Institute of Standards and Technology, the United States of America) standard), and there is an attempt to use it as a reference absorption peak for determining the reference position on the short-wavelength side, in place of the emission line wavelength of the mercury lamp.
However, when measuring the absorption spectrum of a commercial Ho glass filter, it was found that the absorption peak at 241.7 nm as the reference absorption peak overlapped a large absorption peak, on the short-wavelength side, based on an additive for a substrate, and that the reference absorption peak position was shifted from 241.7 nm. Moreover, the shift amount was different for each commercial Ho glass filter.
An example is shown in FIG. 11. FIG. 11 shows the absorption spectra of three commercial Ho glass filters, and shows the range around 241.7 nm. Calibration is performed by the actual analysis device that measures the sample. Accordingly, measurement of the absorption spectrum is also performed by a spectrophotometer for sample measurement, and although the measurement bandwidth is great with the slit width of the exit slit being 8 mm, it is possible to determine the absorption peak wavelength. If the slit width of the exit slit is reduced and the measurement bandwidth is reduced, a sharp peak waveform may be obtained, but the amount of diffracted light transmitted to a cell for measurement is more reduced as the slit width of the exit slit is more reduced. Also, even if the peak waveform is sharp, the absorption peak wavelength of Ho is still affected by the absorption peak due to an additive for a substrate.